Resistor structure for thin film variable resistor

ABSTRACT

A THIN FILM RESISTOR STRUCTURE FOR A VARIABLE RESISTOR IS DESCRIBED. THE STRUCTURE EXHIBITS A LOW CONTACT RESISTANCE AND GOOD TEMPERATURE STABILITY BY EMPLOYING A MULTILAYERED CONSTRUCTION. A TANTALUM NITRIDE THIN FILM LAYER IS FORMED OVER AN INSULATOR SUBSTRATE AND IS COVERED IN TURN WITH A THIN LAYER MADE OF A LOW-RESISTANCE AND ANTI-OXIDATION MATERIAL. THE COMPOSITE MULTI-LAYERED STRUCTURE IS THEN SUBJECTED TO A HEAT TREATMENT TO DIFFUSE OR, SCATTER THE LOW-RESISTIVE AND ANTI-OXIDATION MATERIAL IN A SURFACE REGIONS OF THE TANTALUM NITRIDE. THE FINAL HEAT TREATED STRUCTURE EXHIBITS A LOW NOISE LEVEL WHEN EMPLOYED WITH A BRUSH FOR OBTAINING A VARIABLE RESISTOR. THE HEAT TREATMENT IS FURTHER EMPLOYED TO IMPART A PARTICULAR TEMPERATURE COEFFICIENT OF RESISTANCE TO THE STRUCTURE. SEVERAL EMBODIMENTS AND EXAMPLES ARE SHOWN.

Unitcd States Patent US. Cl. 117--217 18 Claims ABSTRACT OF THEDISCLOSURE A thin film resistor structure for a variable resistor isdescribed. The structure exhibits a low contact resistance and goodtemperature stability by employing a multilayered construction. Atantalum nitride thin film layer is formed over an insulator substrateand is covered in turn with a thin film layer made of a low-resistiveand anti-oxidation material. The composite multi-layered structure isthen subjected to a heat treatment to diffuse or scatter thelow-resistive and anti-oxidation material in a surface region of thetantalum nitride. The final heat treated structure exhibits a low noiselevel when employed with a brush for obtaining a variable resistor. Theheat treatment is further employed to impart a particular temperaturecoefficient of resistance to the structure. Several embodiments andexamples are shown.

This invention relates to a resistor structure suitable for a' thin filmvariable resistor structure to which a slide brush may be slidablyattached.

In recent years, requirements have steadily grown for smaller, moredependable, and lower cost parts for communication apparatus. To meetsuch requirements a number of circuit elements using films haveheretofore been proposed. With regard to variable resistors, effortshave been made to miniaturize and improve the reliability ofconventional variable resistors of the small wire wound type using anadvance wire. For instance, Cu-Ni alloy, and film variable resistorsplaced on substrates made of various films (such as Nichrome thermetgrades) have so far been developed. However, there are still a number ofproblems to be settled before these new resistors can 'be put in use.For example, these thin film variable resistors are usable only within alimited working temperature range of 65 C. to +80 C. They alsoinvariably have a contact resistance of more than 0.2 percent of thetotal resistance of the resistor and exhibit a sliding noise of over 3millivolts. They exhibit material changes in the resistance valuesduring a life test under load, and their power rating is low.

It is known that the resistive material for thin films is made oftantalum nitride to partially improve the defects of conventionalresistors, as described for instance in The Bell System TechnicalJournal, January 1964, pp. 127- 142 and Bell Laboratories Record,OctoberNovember 1966, pp. 305-311. The thin film made of tantalumnitride can readily be prepared by 'an active cathode sputtering processusing a tantalum plate as a cathode. It is possible to provide the thinfilm resistor having a specific resistance within range of 50-5000ail-cm. and a temperature coefficient within the range of 300-+300p.p.m./ C. by suitably controlling the conditions of the sputteringprocess. According to the characteristics of tantalum nitride asmentioned above, a thin film of tantalum nitride provides an excellentresistor both with regard to its reliability and temperature stability.Yet the variable tantalum nitride cannot be put to practical use becauseits specific resist- ICC ance is so high and its surface tends toproduce an insulative material such as tantalum oxide. As a result theconventional tantalum nitride thin film variable resistor has a highcontact resistance and a high sliding noise.

The principal object of this invention is therefore to provide aresistor structure suitable for a thin film variable resistor which hasa low contact resistance and a low sliding noise and which operatesstably over a wide temperature range.

According to this invention, a resistor structure for a thin filmvariable resistor is provided wherein a film of tantalum nitride isformed as a resistor base on one major surface of an insulativesubstrate, such as glass or ceramic plate. A first metal or alloy layerof low resistivity and which is highly adherent to tantalum nitride isformed on the film at the upper surface region of the tantalum nitride.A second metal or alloy layer which has a lower resistivity and agreater anti-oxidation property than the first layer is formed on thefirst layer. The final structure is subjected to a heat treatmentpermitting diffusion and reaction to occur among the layers.

Experimental results have shown that contact resistance and slidingnoise can be decreased by providing a surface of low resistance on theresistor structure for direct contact with a slide brush even if thespecific resistivity of the resistor material is high. For this reason,the resistor structure of this invention has a film of tantalum nitrideand a metal-rich contact layer obtained after forming a metal layer andthus subjecting the film and metal to a heat treatment. The metal of themetal-rich layer needs to be of low resistivity and a low tendency tooxidize in comparison with tantalum nitride.

Variable resistors employing the resistor structure ac cording to thisinvention have a working temperature range of 65 C. to C., small contactresistance and small sliding noise. Moreover the resistor structure ofthis invention can be used for higher rated powers than conventionalthin film tantalum nitride resistors.

To facilitate an understanding of the above-mentioned features, thepresent invention will be described in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a longitudinal cross sectional view of the first embodiment ofthis invention; FIG. 2 is a similar view of the second embodiment ofthis invention; FIG. 3 is a graph showing the temperature coefficientvs. heat treatment temperature characteristic curve of the resistor ofthe second embodiment; FIG. 4 is a longitudinal cross sectional view ofthe third embodiment of this invention; and FIG. 5 is a graphillustrating the relationship between the compound ratio of the thirdembodiment and the temperature coefiicient of resistance.

Referring to FIG. 1, the thin film resistor structure 10 of the firstembodiment of this invention is manufactured by the following process:

A film 12 of tantalum nitride is formed on a substrate of insulativematerial 11 such as glass or ceramics through the sputtering process.Over the surface of the film 12 is further deposited by a vacuumevaporation a thinner film 13 of a metal or alloy which is highlyadherent to tantalum nitride, for example osmium, rhenium, iridium,rhodium, molybdenum, cobalt, manganese, vanadium, nickel, chrome, orNichrome alloy. Next, the structure as a whole is heat-treated fordiffusion and reaction.

The resistor structure obtained through the above mentioned processproduces a metalor alloy-rich layer of-low surface resistance whereinthe metal is scattered at the surface of the tantalum nitride film afterthe heat treatment. A variable resistor employing a slide brush to theresistor structure of this invention has excellent characteristics suchas small contact resistivity and low sliding noise. Furthermore, thesurface of the resistor structure is chemically inactivated. Thesecharacteristics are shown by the following example.

EXAMPLE 1 As an insulator substrate, a ceramic disc having a thicknessof one millimeter and a diameter of 10 millimeters is used. It is washedin the usual manner to cleanly remove oil, grease and other impuritiesfrom the surface. In order to form a tantalum nitride film on thissubstrate, a diode cathode sputtering apparatus using a tantalum plateas the cathode is employed.

The sputtering conditions for this purpose are shown in Table 1.

TABLE 1 Total pressure of vacuum system (Ar+N )4 10* torr Partialpressure of nitrogen gas1.72 10 torr Distance between anode andcathode80 mm. Inter-electrode voltage-3.5 kv.

Under the conditions above tabled and using an evaporation mask, a thinfilm of tantalum nitride of horseshoe shape with a film thickness of3000 A., width of 1 mm., and length of 25 mm. is formed on thesubstrate. Over this film a thinner layer of nickel is deposited to athickness of 150-200 A. by vacuum evaporation using the same evaporationmask as employed for the film of tantalum nitride, thereby forming atwo-layer film structure.

The resistor structure thus formed is heat treated in a vacuum at 400 C.for 3 hours and thereafter in air at 300 C. for 3 hours. Lead terminalsfor this structure are provided by terminals consisting of Nichromealloy layer of a few hundred angstroms and a gold layer of a fewthousand angstroms in thickness, both being successively coated over theresistor structure on both ends thereof. The lead terminals are formedby vacuum evaporation on both ends of the horseshoe-shaped resistorstructure.

The resulting resistor element has a resistance of about 1.2 KS2. Thecharacteristics of the variable resistor are given in Table 2 and weredetermined by using a carbon piece having an area of 0.3 x 1 mm. as aslider brush and applying a pressure of 150 g. to the brush.

TABLE 2 Resistance value 1170 Resis-temp coefficient (p.p.m./ C.)Contact resistance (percent) 0.15 Sliding noise (mv.) 2

The resistance of this structure can be controlled by varying thetemperature for heat-treatments in vacuum and then in air. Thetemperature coetficient of resistance is controllable through avariation of the temperature in vacuo.

Referring to FIG. 2, a thin-film resistor structure 20 is obtained byfurther depositing to the structure of the first embodiment in FIG. 1 alow-resistive and anti-oxidizing layer 14 of metal or alloy of gold,platinum, palladium and silver, and thereafter subjecting the structureto heattreatment. The resistor structure thus attained in his embodimentis highly reliable and high performance may be understood from thefollowing example.

EXAMPLE 2 A horseshoe-shaped thin-film of tantalum nitride which has athickness of 300 A., width of 1 mm. and length of 25 mm. is formed on aninsulator substrate under the same conditions as with the firstembodiment. On the film of tantalum nitride, thinner layers of Nichromealloy and gold are successively deposited through vacuum evaporations toa thickness of 50 A. and 100 A., respectively. The resistor element thusformed consists of three layers, tantalum nitride-Nichrome-gold.

After that, the resistor structure is heat-treated in vacuum at 600 C.for one hour and thereafter in air at 300 C. for three hours. As leadterminals for this resistor element, terminal layers of gold areemployed which may be (Life test results obtained under application of/2W load at C.)

Resistance value (0) 940.5 Temperature coefiicient of resistance(p.p.m.'/

C.) -3l.0 Contact resistance (percent) v 0.01 Sliding noise (mv.) 0.3Life test (percent/2000H) 0.1

For comparison the characteristics of a resistor structure obtainedwithout the heat-treatments in vacuum and in air are shown in Table 4.

TABLE 4 (Life test results obtained under application of /2-W load at 70C.)

Resistance value (0) 850 Temperature coetficient of resistance (p.p.m./

C.) Contact resistance (percent) 0.1 Sliding noise (mv.) 0.2

Life test (percent/1000B) 1 As will be understood from Tables 3 and 4,through the heat treatments the resistor structure is inactivated. Thisimprovement may be explained on the basis of the condition of thesurface of the resistor structure. In other words the low-resistive andanti-oxidation metals are scattered after the heating processes fordiffusion and reaction. Thus, a metal-rich-layer is produced.

In this embodiment, if the structure is not heat-treated in vacuum, thetemperature coefficient of resistance will have a positive value.Moreover, the temperature coefficient of resistance (R) of the structuremay be varied by changing the temperature (T) during the heat-treatmentin vacuum in accordance with the graph shown in FIG. 3. Therefore, thetemperature coefficient of the resistance may be set to any arbitraryvalues by selecting the vacuum temperature of the heat treatment.

In addition, when it is desirable to control the temperature coefficientover a wider range, such control is easily attained by varying thesputtering conditions of Table 1. For instance a larger negativetemperature coefiicient can be obtained by increasing the partialpressure of the nitrogen gas. On the other hand, the temperaturecoefficient can be increased to a positive value by decreasing thenitrogen gas pressure.

Referring to FIG. 4, the thin film resistor structure 30 of the thirdembodiment of this invention is fabricated so that a silver layer 15 anda palladium layer 16 are suc-' cessively evaporated in place of the goldlayer 14 shown in FIG. 2. Then, a rich layer of silver and palladium isproduced on the surface of tantalum-nitride layer throughheat-treatment. According to this embodiment, the resistor structure iscapable of varying its temperature coeflicient with the compound ratiobetween silver and palladium.

EXAMPLE 3 Under the same dimensions and conditions as the Ex-' ample 2,a resistor structure consisting of tantalum-nitride film 12 of 3000 A.thick and having a Nichrome layer 13 of 50 A. thick is prepared. In thisexample, the structure is further subjected to evaporation of a silverlayer 15 of a thickness of 60 A., and a palladium layer 16 of athickness of 200 A. Both ends of the structure are coated by Nichromeand gold for forming the terminal leads.

i TABLE 5 Resistance value 1230 Temperature coefficient of resistance(p.p.m./

C.) +20 Contact resistance (percent) 0.1 Sliding noise (mv.) 1

As mentioned above, the temperature coefiicient of the resistor of thisembodiment can be controlled by varying the vacuum heat treatmenttemperature similarly like thatwith the second embodiment. Furthermore,the coefiicient is controllable through the variation of the ratio ofsilver and palladium as graphically shown in FIG. E.

The same temperature coeflicient control is obtained by evaporating onthe surface of the tantalum nitride film a previously preparedsilver-palladium alloy followed by the heat-treatment in the manner ofExample 2.

As has been described, the resistor structure for thin film variableresistors made according to the present invention has a low contactresistance, small sliding noise, excellent temperature coefiicient, andhigh reliability, as compared with those of conventional thin filmresistors.

We claim:

1. A resistor structure for a thin film variable resistor comprising aninsulator substrate and a resistor film adhering to a surface of saidsubstrate and consisting essentially of tantalum nitride, said filmhaving at its outer surface a region into which a low-resistance andantioxidation material is scattered.

2. The resistor structure as recited in claim 1 wherein said material isselected from the group consisting of 0smium, rhenium, iridium, rhodium,molybdenum, cobalt, manganese, vanadium, nickel, chromium andnickelchromium alloy.

3. The resistor structure as recited in claim 2 wherein said regionfurther includes a material selected from the group consisting of gold,palladium, silver, platinum and silver-palladium.

4. The resistor structure as recited in claim 1 wherein said materialincludes nickel-chromium alloy and a material selected from the groupconsisting of gold, palladium, silver, platinum and silver-palladium.

5. A resistor structure for a thin film variable resistor comprising aninsulator substrate, a resistive thin film layer consisting essentiallyof tantalum nitride placed on said substrate, and a second thin filmlayer of low-resistive and anti-oxidation material placed over saidresitive thin film layer, said second thin film layer being scatteredthroughout a surface region of said thin film resistive layer.

6. The device as recited in claim 5 wherein the second .thin film layerof low-resistive and anti-oxidation material is selectively scatteredthroughout said surface re gion to control the temperature coefiicientof resistance of said resistor structure.

7. The device as recited in claim 5 wherein said second thin film layercomprises a pair of thin film metallic conductive layers overlying oneanother and the first thin film layer, with the layer adjacent the firstthin film layer being substantially chosen for strong adherence to saidfirst thin film layers and the uppermost of the pair of layers beingselected to impart an anti-oxidation characteristic to the structure.

8. A method for forming a thin film resistive structure for a variableresistor comprising the steps of depositing on a substrate a thin filmlayer of resistive material consisting essentially of tantalum nitride,depositing on said resistive material a thin film layer of metalliclow-resistive and anti-oxidation material, and applying heat for apredetermined duration of a range of 1-3 hours to said compositemulti-layered structure and raising the temperature thereof to apreselected value of a range of 400 C.-600 C. to form a metal-richcontacting layer in a surface region of said resistive material.

9. The method as recited in 8 wherein said metallic low-resistive andanti-oxidation layer is formed with a thickness less than the thicknessof said resistive thin film layer for diffusion and scattering of saidmetallic layer into said resistive layer.

10. The method as recited in claim 9 wherein said metallic low-resistiveand anti-oxidation material depositing step further includes depositinga first thin film metallic layer of conductive material over saidresistive material, and

depositing a second thin film metallic layer of conductive material oversaid first thin film metallic layer.

11. The method as recited in claim 10, wherein the heating step includesthe steps of heating said composite multi-layered structure in vacuumfor a preselected time of a range of 1-3 hours and at a selectedtemperature of a range of 400 C.-600 C. and heating said compositemulti-layered structure in air for a selected time of approximatelythree hours and at a selected temperature of approximately 300 C.

12. A method for forming a thin film resistive structure for a variableresistor comprising depositing on an insulative substrate a thin filmlayer of tantalum nitride, depositing on said tantalum nitride layer athin film layer of metallic conductive and oxidation-resistant material,applying heat for a predetermined duration of a range of l-3 hours tosaid composite multi-layered structure and raising the temperaturethereof to a preselected value of a range of 400 C.600 C. to form ametal-rich layer in a surface region of said tantalum nitride layer.

13. The method as recited in claim 12 wherein the thin film layer oversaid tantalum nitride is deposited with a smaller thickness than saidtantalum nitride layer.

14. The method as recited in claim 12, wherein said heating stepincludes heating the composite multi-layered structure in a vacuum forsaid preselected time at said preselected temperature, and furtherincludes heating the composite multi-layered structure in air for apreselected time of approximately three hours at a preselectedtemperature of approximately 300 C.

15. A method for forming a thin film resistive structure for a variableresistor comprising the steps of depositing on an insulative substrate athin film layer of tantalum nitride, depositing on said tantalum nitridelayer a first thin film layer of a material selected from the groupconsisting of osmium, rhenium, iridium, rhodium, molybdenum, cobalt,manganese, vanadium, nickel, chromium, and nickelchromium alloy, andapplying heat to said multi-layered structure for a predeterminedduration of a range of l-3 hours and at a selected temperature of arange of 400 C.600 C. to form a metalrich contacting layer in a surfaceregion of said tantalum nitride.

16. The method as recited in claim 15 and further including the step ofdepositing after the deposit of said first thin film material layer andbefore the heating step a second thin film layer over said first thinfilm layer,

said second thin film layer being made of a material selected from thegroup consisting of gold, palladium, silver, platinum andsilver-palladium.

17. A method for forming a thin film resistive structure with a desiredtemperature coeflicient of resistance variable resistor comprising thesteps of depositing on a substrate a thin film layer of resistivematerial consisting essentially of tantalum nitride, depositing on saidresistive material a plurality of metallic layers selected to provideoxidation resistance, adherence to said resistive material,

and wherein a pair of metallic layers have thicknesses selectedcommensurate with a desired temperature coefficient of the compositemulti-layered structure, and applying heat to said compositemulti-layered Structure for a predetermined duration of a range of 1-3hours and to raise the temperature thereof to a preselected level of arange of 400 C.-600 C. to form a metal-rich contacting layer in asurface region of said resistive material and further control saidtemperature coeflicient of resistance.

18. A method for forming a thin film resistive structure for a variableresistor comprising the steps of depositing on an insulator substrate athin film of tantalum nitride, depositing over said tantalum nitridelayer a thin film layer of Nichrome, depositing over said Nichrome thinfilm layers of silver and palladium in a predetermined ratio selectedcommensurate with a desired temperature coefiicient of resistance, andapplying heat for a predetermined duration of a range of l-3 hours tosaid corn posite multi-layered structure and raising the temperaturethereof to a preselected value of a range of 400 C.600

C. to form a metal-rich contacting layer in a surface re-. gion of thetantalum nitride layer and to further deter mine the temperaturecoeflicient of resistance of said structure.

References Cited UNITED STATES PATENTS 3,010,850 11/1961 Colbert 'et al...1l7 2l7X 3,112,222 ll/1963 Alger .l l17217 3,242,006 3/1966Gerstenberg 117-201 WILLIAM L. JARVIS, Primary Examiner U.S. Cl. X.R.

